The present invention relates to a printer for printing out an image on a sheet by wrapping a stencil formed with perforations representative of the image around an apertured drum, and supplying ink to a sheet via the drum and stencil while pressing the sheet against the stencil by a press roller.
In a printer of the type described, a clamper is provided on a drum for retaining one edge of a stencil wrapped around the drum. As the drum is rotated, the stencil is rotated together with the drum while being pulled by the clamper. It is likely that the stencil so retained on the drum is pulled in the opposite direction to the direction of rotation of the drum due to various factors. Specifically the impact which a press roller exerts on contacting the drum via the stencil and a sheet is one factor. Another factor is the loads ascribable to the rotation of the press roller and an inking roller which is disposed in the drum. Still another factor is the load exerted by a sheet feed device while the stencil is transported by the drum together with a sheet by being held between the drum and the press roller. As a result, when a number of printings are produced with the single stencil wrapped around the drum, the edge of the stencil retained by the clamper is apt to slip out of the clamper little by little. Then, the stencil is dislocated on the drum to have the print start position thereof changed, preventing an image from being accurately printed out in a predetermined position on a sheet.
Various attempts have been made to prevent the stencil wrapped around the drum from being released from the clamper. For example, the clamper may be implemented by a metallic plate and a rubber plate having a magnet. As the force of the magnet provided on the rubber plate is increased, the clamper retains the stencil more positively on the drum. However, the problem with this approach is that the overall size of the clamper is increased and, in addition, a great force is necessary for the metallic plate and rubber plate to be moved toward and away from each other. One of the two members for clamping the edge of the stencil may be constituted by a high friction member, as disclosed in Japanese Utility Model Laid-Open Publication No. 31830/1986. This kind of scheme, however, is apt to damage the stencil since it forcibly prevents the stencil from slipping out of the clamper by the high friction member, i.e., it exerts a substantial load on the stencil. Moreover, the high friction member is apt to catch a sheet, bringing about a jam problem.
Japanese Utility Mode Laid-Open Publication No. 174407/1979, for example, teaches a press roller which is rotatably supported by one end of a rotatable arm and pressed against the drum by a spring or similar resilient member. The time when the press roller begins to press itself against the drum and the time when the former leaves the latter are determined by the profile of a cam with which a cam follower is engaged. The cam follower is mounted on a support member which supports the press roller. Usually, the cam is configured such that the press roller begins to contact the drum, i.e., the stencil at a particular time associated with a position slightly ahead of the leading edge of the perforated portion of the stencil, i.e., a perforation start point. However, the problem is that the press roller oscillates due to bounce when it hits against the rigid drum under the action of the spring. Such a bounce or oscillation will effect an image little so long as the drum is rotated at low speed, since the rotation or displacement of the drum up to the time when the bounce or the oscillation terminates is small. Nevertheless, the above-mentioned displacement of the drum increases with the increase in the rotation speed of the drum. During this period of time, the pressure exerted by the inking roller on the inner periphery of the drum and the pressure exerted by the press roller on the stencil and sheet are not sufficient, obstructing a stable operation. Specifically, ink fails to reach some portions of a sheet associated with the locus of the bounce or oscillation of the press roller. Then, an image is locally lost in such portions of the sheet while the resulted image density distribution is irregular. Moreover, when the press roller is to abut against the drum which is rotating at high speed, the cam is also rotated at high speed. Therefore, if the cam has a profile which causes the press roller into contact with the drum instantaneously, the press roller produces noise on hitting against the drum in the portion of the cam where the profile sharply changes.
The drum is made up of an apertured hollow cylindrical support and a mesh screen covering the outer periphery of the support. The outer periphery of the cylindrical support has a permeable zone for passing ink from the inking roller therethrough to the stencil via the mesh screen, and a non-permeable zone. The mesh screen is affixed at the leading edge thereof to the support by screws or similar fastening means. The trailing edge of the mesh screen is constantly pulled by, for example, a spring. As the printing operation is continued with a stencil wrapped around such a drum to produce a great number of printings, the ink intervening between the stencil and the support of the drum, i.e., impregnated into the mesh screen is sequentially forced to the rear by the press roller until it bulges out from between the trailing edge of the stencil and that of the screen. This part of the ink smears not only the outer periphery of the drum but also the trailing edge of the sheet and the press roller. The press roller so smeared would in turn smear the back of the sheet. At the same time, the ink sequentially forced to the rear by the press roller also enters the interstice between the support and the trailing edge of the screen to bulge out from the trailing edge of the screen. This smears the outer periphery of the support and, therefore, the sheet. This problem is especially pronounced when use is made of soft and highly infiltrative ink.
An approach to prevent ink from bulging out from between the trailing edges of the stencil and mesh screen is proposed in, for example, Japanese Patent Publication No. 28670/1985. This approach, however, cannot prevent the ink having entered the interstice between the support and the trailing edge of the screen from bulging out and depositing on the outer periphery of the support. Although the trailing edge of the screen may be adhered to the outer periphery of the support to eliminate such an occurrence, adhesion is not desirable since the trailing edge of the screen is constantly pulled to the rear by a spring or similar biasing means, as stated above.
On the other hand, the ink from the inking roller disposed in the drum is supplied to the stencil via the apertures formed in the permeable zone of the support. Part of this ink sometimes leaks to the outside at opposite sides of the drum to smear various parts and elements built in the printer. Japanese Patent Publication No. 54039/1984 discloses a countermeasure to this kind of contamination. Specifically, a scraper is provided on the inner periphery of the drum and inclined relative to the direction of rotation of the drum so as to collect the ink bulging out at the position where the inking roller and the drum contact each other. More specifically, as the ink existing between the inking roller and the drum is forced out from the end of the inking roller when the press roller presses itself against the drum, the scraper guides such ink inwardly in the axial direction of the inking roller. However, the flow of ink is not stable when the ambient temperature is low and the printing operation is effected at high speed. It follows that the ink deposits on the scraper and is scattered around in masses in the event of printing. As a result, the scraper fails to serve the expected function and causes irregularities to occur in a printed image.